The Federal Aviation Administration (FAA) of the U.S. Government and other bodies have requested designs for enclosures for various types of antennas to be used at airports. An important requirement for these enclosures is the ability to withstand repeated jet blasts of 150 m.p.h. from aircraft and sustained winds. A jet blast may have a duration of up to one minute, may be from an angle up to 15.degree. from horizontal and may occur up too 500 times per month. However the enclosure must shatter on impact by a colliding light aircraft traveling at about 75 m.p.h. Moreover, only minimum damage upon the aircraft can be inflicted. To this end, upon impact the enclosure must shatter into small pieces e.g., pieces having a maximum span of at most four feet. These criteria result from a plan to construct antenna towers and other structures very close to and along airport runways, such as for use with improved instrument landing systems. The location of these structures necessitates the frangibility of the enclosures. I am currently unaware of any enclosures which are specifically designed to be frangible and to shatter upon impact yet withstand large distributed loads.
A radar antenna in such an enclosure must be accessible by installers and maintenance personnel, thus, a floor system is required therein to allow personnel to work inside the enclosure. Such a floor system needs to support primarily vertical loads of personnel and equipment but also needs to break up easily into separate parts when subjected to impact loads from a collision. These impact loads normally involve large horizontally directed forces. A typical load of personnel and equipment on a floor which must be supported by the floor system is about 100 pounds per square foot in the vertical direction. A substantially horizontal impact load of 700 ft./lbs. per breakage area must also cause failure of the floor system.
An access door is also needed in the enclosure to permit personnel access. Such doors need to be designed so that the frangibility of the enclosure is not compromised. Furthermore, a door should cause minimal interruption of the enclosure surface.
In an enclosure for an antenna, the antenna window must also incorporate an anti-icing system because ice absorbs radar signals. Such a system uses heating wires for which the size, material and spacing need to be selected in order to ensure proper antenna performance and anti icing without compromising the frangibility of the enclosure.
Typical antenna and building enclosures for airport use are made of a plurality of panels or other structional elements which are attached together to form a housing. However, there are currently no panels or housing construction designed to fail in a manner as described above.
The details of the enclosure design were not addressed by the request from the FAA. The design of an adequate frangible enclosure needed to consider the shape of the enclosure so that drag forces could be reduced by minimizing its size, configuration, the materials of construction, and the effect of the construction on antenna performance.
Accordingly, it is an object of the present invention to provide a frangible enclosure, for housing an antenna or other use, with low resistance to localized impact, but having high resistance to distributed loads, such as those from wind.
More particularly, it is an object of the invention to provide an enclosure that will withstand repeated jet blasts from large aircraft, but shatter on impact from a colliding light aircraft, while inflicting minimum damage upon the aircraft.
Another object of the present invention is to provide a frangible enclosure which upon localized impact shatters into small pieces, e.g. pieces having a dimension of at most four feet.
It is another object of the present invention to provide a frangible enclosure for an antenna, which enclosure has an antenna window with anti-icing elements which do not compromise the frangibility of the enclosure.
A further object of the present invention is to provide a frangible enclosure that is easily fabricated, transported and handled.
Another object of the present invention is to provide a frangible enclosure having accessibility for personnel and which does not hinder the frangibility of the enclosure.
More particularly, it is an object of the present invention to provide a flooring system for access and support for maintenance personnel, which supports primarily vertical loads of personnel and equipment but also breaks up into separate pieces under normally horizontal impact loads.